The high-temperature (.about.288.degree. C.) water coolant in a boiling water reactor (BWR) is highly oxidizing due to dissolved, radiolytically produced chemical species, such as oxygen and hydrogen peroxide. These molecules are generated as the water passes through the reactor core and is exposed to very high gamma and neutron fluxes. The dissolved oxidants create a relatively high electrochemical potential (ECP) for structural materials in the coolant. Because of the high ECP, reactor structural materials in contact with the coolant, such as stainless steels and nickel-base alloys, can suffer intergranular stress corrosion cracking (IGSCC). This can limit the useful lifetime of reactor components, such as piping and pressure vessel internal structures, or result in a large inspection and repair cost in an effort to mitigate IGSCC effects in nuclear plants.
A number of countermeasures have been developed to mitigate IGSCC in BWRs. Of the various mitigation strategies, reducing the environmental aggressiveness (i.e., oxidizing potential, or ECP, of the coolant), can provide the best approach, since the coolant contacts all the potentially susceptible surfaces of interest. A primary strategy to reduce the ECP to some benign value has been to add hydrogen gas to the reactor feedwater in sufficient quantity that it is available to chemically combine, in the presence of a radiation field, with the dissolved oxygen and hydrogen peroxide to form water, thereby reducing the ECP below the IGSCC threshold value. Another strategy is directed towards providing IGSCC protection of selected high cost-impact reactor systems, such as piping, by reducing the ECP of these particular systems by inserting a catalytic recombiner upstream of the piping, or other system requiring IGSCC protection. The recombiner facilitates the reaction of a small (stoichiometric) hydrogen addition with the dissolved oxidants, and the oxidizing power of the water exiting the recombiner is reduced below the IGSCC threshold value downstream of the recombiner, up to the point where the water either mixes with higher oxidant-containing coolant, or again passes through the reactor core where radiolysis call recur.
The origin of ECP is based in the fundamental nature of metals, which are characterized by atoms consisting of an equal number of positive and negative charges (protons and electrons). Metals are formed from naturally occurring ores, or oxides, in which the metallic atoms are ionized. In the refining process, high energy and strongly reducing conditions are supplied to force the metallic ions in the ore to become a neutral metal by accepting additional electrons.
In subsequent use, however, metal atoms attempt to reject the added electrons to return to the lower energy (natural) state occurring in ores. If the metal is in contact with an electron acceptor, such as water containing dissolved oxidants, then electronic transfer from the metal to the acceptor is energetically favored, and an oxidation reaction (corrosion) can occur. The electrochemical potential is a measure, under certain fixed conditions, of the thermodynamic tendency for a metal to lose electrons and corrode. Forcing electrons into a metal contained in an oxidizing environment reverses this tendency and prevents, or inhibits, corrosion.
When a metal is in contact with an oxidizing solution, the ECP is a measure of the thermodynamic tendency for the metal atoms to ionize and enter the solution, leaving the metal with a net surface charge. This charge distribution is balanced by ions in the solution, which rearrange themselves in response to the electric field produced by the surface charge density. A boundary layer charge distribution results, and a potential difference exists across the boundary layer, between the metal surface and the neutral bulk solution. This potential difference can be measured, if combined with another "half-cell" electrode forming a crude battery. If the other half-cell accepts electrons, a corrosion reaction is allowed; if it supplies electrons to the metal, no corrosion can occur.
ECP is directly related to intergranular attack in thermally sensitized metals (IGSCC), if the dissolved oxidant concentration is sufficient to provide the necessary electrochemical driving force for this type of corrosion reaction. Typically, the ECP of austenitic stainless steels in the BWR piping coolant environment is about 100 mV on the standard hydrogen electrode (SHE) scale. The core and internals regions of the reactor are even more oxidizing due to higher levels of dissolved oxygen and hydrogen peroxide, produced radiolytically as the coolant flows through the high radiation fields of the core. Typical ECP values of 250 mV (SHE) are encountered in the reactor. Empirically, it is known that a threshold exists for onset of IGSCC, depending on the condition of the metal. If thermally sensitized, the threshold is -230 mV (SHE); if non-thermally sensitized, the threshold for irradiation-assisted IGSCC is -140 mV (SHE). Therefore, all strategies to date are based on lowering ECP below the threshold for IGSCC, either globally by addition of hydrogen gas to the reactor feedwater, or locally by promoting catalytic recombination of hydrogen and oxygen/peroxide dissolved in the coolant.
Both techniques involving hydrogen addition have the effect of reducing the oxidizing environment in contact with stainless steel piping and structural members. Many years of laboratory and reactor testing and analysis have led to the conclusion that these techniques are more or less effective, but massive additions of hydrogen required to globally suppress ECP have serious side effects that are undesirable in practice. For example, radiochemistry effects that produce the isotope N-16 in excessive amounts increase the radiation burden of the reactor system. Catalytic recombiners are expensive, bulky and add to the reactor pressure drop. Their effectiveness in suppressing oxidant concentrations is yet to be fully demonstrated. Therefore, alternative methods are required to lower ECP directly, without expensive hydrogen injection systems and catalytic recombiners.